So a lot of people were shocked to see that Pluto was a not a planet in our solar system anymore. Of course the question is why? Why did all of a sudden Pluto changes from a planet in our solar system to a dwarf planet? Well according to NASA, In 2003, an astronomer discovered a new object beyond Pluto. The astronomer thought he had found a new planet. The object that he saw was larger than Pluto. He found the object that we know today to be Eris. Since its discovery, astronomers have determined that Eris’s size is approximately 2,600 km. With Eris being larger and from the same ice/rock mixture, the concept that we have nine planets in our solar system began to fall apart. Finding Eris caused other astronomers to talk about what makes a planet a planet. So astronomers decided they would make a final decision about the definition of a planet at the International Astronomical Union. The decision was that for an object to be a planet, it needed to meet these three requirements defined by the IAU and they were:

  • It needs to be in orbit around the Sun
  • It needs to have enough gravity to pull itself into a spherical shape
  • It needs to have “cleared the neighborhood” of its orbit

Well Pluto meets the first two requirements. It’s problem is the last one. What the IAU means by “cleared the neighborhood”, it means that as planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other smaller objects, they either consume them, or sling them away with their gravity. Pluto’s gravity isn’t strong enough so Pluto cannot push or consume anything in its path. It cannot sweep up asteroids, comets, and other debris to clear a path along its orbits. Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. Of course there is still hope of Pluto rejoining us in the solar system! There are still many objects with similar size and mass to Pluto lying around in its orbit so if Pluto does crash into many of them and gains mass, it will probably come back to the solar system. Until then however it will remain a dwarf planet.